In fluid flow machines bladed rotors comprising a rotor disc bearing aerofoil blades around its rim are commonly used. Such rotors are vulnerable to damage to the rotor disc rim causing blades to break off of the disc or the disc itself to break up. Such damage can be caused by erosion by the fluid flow itself or impact damage by solid foreign objects carried in the fluid flow.
These problems are particularly pronounced in the turbine and compressor rotors in gas turbine engines because of the very high rates of rotation involved. As a result centrifugal loads are very large and failure of the rotor disc or blade loss can be catastrophic because of the high kinetic energy of the released blade or disc fragments.
In addition these high rates of rotation and the generally high gas flow velocities within the engine make the chances of erosion or foreign object damage more likely than in rotors subjected to less extreme conditions, this is particularly true of turbine rotors which operate at high temperatures in a very high temperature gas flow.
The very high temperature of the gas flow can also indirectly cause damage to the disc due to the stresses produced by differential thermal expansion, because the disc rim will be heated by the gas flow to a much higher temperature than the main bulk of the disc.
One known method of protecting the rotor rim from these problems is to coat it with a layer of material less susceptible to damage than the basic rotor material and having a low thermal conductivity. The choice of rotor material generally cannot be made based on damage resistance and capacity to endure temperature differentials alone but must be a trade off between these and other properties such as strength and density, but the use of a coating to protect the disc from impact and insulate it to reduce temperature differences allows the disc material to be selected based only on these other properties.
The use of such a coating has two main drawbacks, firstly the problem of ensuring that the coating does not separate from the disc under centrifugal and differential thermal expansion loads and secondly, if a blade is damaged it will be more difficult to remove and replace it because this will generally require that at least part of the coating also be removed and replaced, a demanding operation.
This invention was intended to provide a rotor at least partially overcoming these problems.